Process for preparing aspartic acid from ammonium aspartate, and continuous process for preparing polysuccinimide involving such process

ABSTRACT

The invention relates to a process for the preparation of aspartic: acid by decomposition of ammonium aspartate wherein a concentrated aqueous solution of ammonium aspartate is continuously introduced into a diluent in which water has a low solubility, which diluent is maintained at a temperature at which ammonium aspartate decomposes into aspartic acid and ammonia, and wherein the water and the formed ammonia are continuously removed. The invention also relates to a process for preparing polysuccinimide starting from fumaric acid or maleic acid and ammonia involving said process for preparing aspartic acid from ammonium aspartate.

This invention relates to a (continuous) process for the transformationof ammonium aspartate into aspartic acid, and to a continuous processfor the production of polysuccinimide by a polycondensation step ofammonium aspartate thus obtained.

According to the state of the art, two major routes exist to preparepolysuccinimide

the polycondensation of maleic anhydride with ammonia, resulting in apolycondensate composed of D- and L-anhydroaspartimide building blocks.Typical examples of this process are described in a.o. EP 256366, EP578451, U.S. Pat. No. 5,410,017 or EP 612784.

the polycondensation of aspartic acid or precursors thereof, such asammonium aspartate. This reaction is performed with or without catalystaddition. Typical examples of this process are described in a.o. U.S.Pat. No. 3,846,380, EP 406623, U.S. Pat. No. 5,219,986, WO 9214753, orEP 578448.

Polysuccinimide is used as a starting material for the production ofpolyaspartic acid, which finds application a.o. as Ca scavengers indetergents.

When using maleic anhydride as the starting compound, a D,L-polyasparticacid is obtained after the alkaline hydrolysis of the correspondingpolysuccinimide. This polycondensate has a less good biodegradabilitythan the polycondensates of L-aspartic acid.

The L-aspartic acid used in the preparation of L-polyaspartic acid isobtained via the bioconversion of fumaric or maleic acid, or saltsthereof. During these bioconversion reactions ammonium fumarate iscontacted with an aspartame enzyme system, while ammonium maleate iscontacted with a maleate isomerase/aspartase enzyme system. Thesetransformations can be conducted in batch or continuously. A continuousprocess is e.g. described in GB 2084155, EP 89165 or DE 2835874, whereammonium fumarate is reacted over a column containing an immobilisedaspartase-producing micro-organism. The resulting ammonium aspartatesolution can then be transformed into an aspartic acid solution followedby the crystallisation of the corresponding acid.

Several methods are described for the recovery of crystalline asparticacid. The standard method for obtaining crystalline aspartic acidcomprises the addition of sulfuric acid to the ammonium aspartatesolution. This method allows the formation of ammonium sulfate andaspartic acid. After crystallisation of the aspartic acid the ammoniumsulfate residue is recovered and used as fertiliser.

Another method for recovering aspartic acid is described in EP 588674and consists in the addition of fumaric acid to the ammonium aspartatesolution. The major advantage of this method resides in the fact that noby-product is formed because ammonia is recovered as ammonium fumaratewhen using fumaric acid as the neutralising agent.

The major disadvantage of this method resides in the fact that thetransformation of ammonium aspartate into ammonium fumarate is timeconsuming and discontirnous.

An improvement of the above process is described in EP 678499. Thisapplication describes the use of an alcoholic solution of fumaric acidwhich is added to an ammonium aspartate solution. The formation ofaspartic acid crystals indeed takes less time but a residual solution ofammonium fumarate in aqueous alcohol is obtained. The alcohol, such asethanol or methanol, must first be removed before the ammonium fumaratesolution can be further processed to ammonium aspartate.

A further method for obtaining aspartic acid out of ammonium aspartateis described in JP 07/330696. Here the ammonium aspartate solution isheated which results in the decomposition of the ammonium salt intoaspartic acid and ammonia, which is evaporated together with the water.This method is quite energy consuming (because hot water has to besupplied continuously and then evaporated) while the ammonium aspartateis not completely converted into aspartic acid.

The conversion of ammonium aspartate into polysuccinimide is forinstance described in DE 4429108. This patent application discloses theformation of polysuccinimide by evaporating a solution of ammoniumaspartate at 80° C. to dry, followed by a 4 hour heat treatment at180-220° C. During this heating a foam is produced which is milled to afine powder. This powder is then further polycondensed to the finalpolysuccinimide.

Continuous methods for preparing polysuccinimide from aspartic acid,with or without catalyst are described in e.g. U.S. Pat. No. 5,449,748,WO 9605241, EP 646615 and EP 578449.

From the prior art it can thus be concluded that a continuoustransformation of fumarate into aspartate is known, and also acontinuous transformation of aspartic acid into polysuccinimide. Acontinuous transformation of ammonium aspartate into aspartic acid wouldtherefore be very interesting because then a continuous process forobtaining polysuccinimide out of fumaric acid would become possible.

Applicants have now succeeded in the development of a continuous processfor preparing aspartic acid from ammonium aspartate and hence forpreparing polysuccinimide out of fumaric acid or maleic acid.

The invention thus specifically relates to a process for the(continuous) preparation of aspartic acid by decomposition of ammoniumaspartate, in which a concentrated aqueous solution of ammoniumaspartate is constantly introduced into a diluent in which water has alow solubility, which diluent is maintained at a temperature at whichammonium aspartate decomposes into aspartic acid and ammonia and inwhich the water and the formed ammonia and aspartic acid arecontinuously removed, preferably at a rate substantially correspondingto the rate of introducing the ammonium aspartate solution into thediluent.

According to a preferred feature of the invention the aqueous solutionhas a concentration of at least 25% by weight of ammonium aspartate.More concentrated solutions, in particular solutions with at least 50%(such as 50-60%) by weight of ammonium aspartate are more economical andtherefore prefered.

According to another preferred aspect of the invention the solubility ofwater in the used diluent is less than 2% by weight, preferably lessthan 1%. Its temperature is appropriately maintained at a temperatureabove 130° C., preferably between 130 and 160° C., most preferablybetween 140 and 150° C.

In selecting the diluent, preference is to be given to those having aboiling point of at least 165° C., preferably between 190 and 300° C.Particularly suitable classes of solvents are higher alcohols, higheraliphatic hydrocarbons, higher aromatic hydrocarbons, higher alkylethers and high boiling aromatic ethers.

Preferred diluents are decanol, dodecanol, decaline, tetraline, andphenyl linear or branched C₁₀-C₂₀-alkanes.

According to a further preferred feature of the invention the rate ofintroduction of the aqueous ammonium aspartate solution and/or the rateof removal of the water, ammonia and aspartic acid is such that theformation of a twoliquid phase system is prevented.

According to still another preferred feature of the invention thecontent of aspartic acid crystals in the reaction medium during thecontinuous process is at least 10% by weight of the amount of diluentused.

The invention also specifically relates to a continuous process forpreparing polysuccinimide starting from fumaric or maleic acid andammonia, involving the said continuous process for preparing asparticacid from ammonium aspartate.

This continuous process for preparing polysuccinimide according to theinvention comprises the following process steps:

reaction of fumaric or maleic acid with ammonia to prepare ammoniumfumarate or maleate,

continuous bioconversion of the ammonium fumarate or ammonium maleaterespectively into an aqueous ammonium aspartate solution, usingimmobilised aspartase or fumarate isomerase/aspartase respectivelycontaining micro-organisms,

concentrating the aqueous solution,

continuously introducing the concentrated aqueous solution of ammoniumaspartate into a diluent in which water has a low solubility, whichdiluent is maintained at a temperature at which ammonium aspartatedecomposes into aspartic acid and ammonia,

continuously removing water and the formed ammonia and aspartic acid ata rate substantially corresponding to the rate of introducing theammonium aspartate solution into the diluent, whereas the ammonia isrecovered for use in the conversion of fumaric or maleic acid intoammonium fumarate or maleate,

thermal polycondensation of the obtained aspartic acid intopolysuccinimide.

It is supposed that when introducing the concentrated ammonium aspartateinto the heated diluent, the water present immediately dissolves in thediluent while the ammonium aspartate solidifies. The temperature atwhich the concentrated ammonium aspartate solution is introduced intothe diluent is high enough to allow the decomposition of the ammoniumsalt into ammonia and aspartic acid. The ammonia together with the wateris continuously removed at a speed which is about equal to the speedwith which water is added to the diluent when feeding with concentratedammonium aspartate. The ammonia-solution thus obtained is used toneutralise fumaric acid or maleic acid used as starting product for thebioconversion step.

As indicated above diluents used in the processes according to theinvention are characterised in that the solubility of water in them isbelow 2%, preferably below 1% W/W. The reaction temperature at whichammonium aspartate is decomposed is between 130 and 160° C., preferablybetween 140 and 150° C.

It is essential that no two-phase system of liquids is formed during thedecomposition of ammonium aspartate. If excess water is not removed fastenough, then a highly concentrated ammonium aspartate melt phase isformed which causes problems of agglomerating, of further dewatering andincomplete decomposition of the ammonium aspartate. The aspartic acidobtained under such conditions is more difficult to transform into thepolysuccinimide polycondensate.

The diluents used in the decomposition of ammonium aspartate do have aboiling point of at least 165° C. More preferably, the boiling point ofthe diluent is situated between 190° C. and 300° C.

Typical diluents which can be used are higher alcohols, higher aliphatichydrocarbons, higher aromatic hydrocarbons, higher alkyl ethers, highboiling aromatic ethers etc.

Among the higher alcohols, C8-C14-alcohols can be used, decanol anddodecanol being preferred. Suitable higher aliphatic hydrocarbons aree.g. C10-C20-hydrocarbons, decaline or tetraline, while among the higheraromatic hydrocarbons phenyl linear or branched higher alkanes can beused.

The above mentioned diluents are preferably substanstially free of waterat the moment that the reaction is started.

The process according to the invention is schematically described havingreference to FIG. 1.

In a first step fumaric acid (respectively maleic acid) is neutralisedwith fresh ammonia and/or ammonia recovered from the decomposition step.The thus obtained ammonium fumarate solution (respectively ammoniummaleate solution) is then continuously fed to a reactor (1) in which anaspartase containing micro-organism is immobilised on an inert carrier.The ammonium aspartate solution is then continously concentrated bymeans of evaporation means to an aqueous solution of at least 50% d.s.,preferably at least 60% d.s. (2).

Falling film evaporators, Multiple Vapour Recompression (MVR)evaporators or multistep-evaporators are particularly suited for thisconcentration step.

This concentrated ammonium aspartate stream is then continuously pumpedinto the reaction vessel (4). Before introduction to the reactionvessel, the concentrate can be heated to about 100° C. via a steamheated heat exchanger (3).

The reaction vessel (4) comprises a diluent which is free of dissolvedwater and of which the temperature is constantly held at 130°-160° C.,preferably at 140°-150° C. The concentrated ammonium asparate is nowcontinuously fed to the diluent at a rate sufficiently low to preventthe formation of a two-phase liquid system Under these conditions, theammonium aspartate decomposes into ammonia and the corresponding acid,while water and ammonia are continuously removed.

The process can be carried out under normal pressure but may also beperformed under reduced or encreased pressure.

The aspartic acid formed is continuously removed at a rate correspondingwith the quantity of ammonium aspartate fed. The total quantity ofaspartic acid present in the reaction vessel (4) is preferably at least10% by weight with respect to the amount of diluent used. When startingthe continuous process this amount of aspartic acid cristals ispreferably pre-introduced in the reaction vessel.

The decomposition of the ammonium aspartate into aspartic acid andammonia can also be performed in a cascade type of reactor.

From the reaction vessel (4) a stream of aspartic acid crystals iscontinuously removed and separated from the diluent in a separationdevice (5). The diluent recovered during this separation step isrecirculated to the reaction vessel (4). Separation devices which can beused here are e.g. rotating vacuum filters, continuous belt filters,centrifuges, decanters, hydrocyclones etc . . . .

The separated aspartic acid crystals are then continuously dispersedinto a diluent which is heated to at least 180° C. Here the return fromthe polysuccinimide separation step (7) is very suitable for thatpurpose. The dispersion of aspartic acid crystals in the diluent is thentransferred in a polycondensation reactor (6).

The polycondensation step can be performed in one step but a multistepcascade process is preferred.

A possible polycondensation reactor is shown in FIG. 2. Thisinstallation comprises two reaction vessels A and B which are bothheated at different temperatures. Both the reaction vessels arecontinuously fed and are working in cascade. Condensation water iscontinuously removed and condensed in e.g. a heat exchanger. From theoutlet of reactor B, polysuccinimide, dispersed in diluent, iscontinuously removed at a rate corresponding to the feeding rate ofreactor A.

Reactor A is preferably heated to a temperature of at least 200° C.Reactor B has a temperature which is at least 20° C. above thetemperature in reactor A.

The polysuccinimide is separated from the diluent and can them befurther processed into polyaspartic acid using the generally knownmethods. The diluent is used to disperse the aspartic acid which is fedto reactor A.

The cascade process as described in FIG. 2 should be considered as theillustration of a principle. Also installations comprising three of morereactors in cascade can be used for that purpose. The difference inreaction temperature between the first reactor and the last one ispreferably at least 30° C.

EXAMPLE 1

In a 1 liter stirred reactor equipped with a rectification column andcooler, a mixture of 50 g of aspartic acid crystals in 417 g of decanolwas heated to about 150° C. Then a solution of ammonium aspartate (60%W/W) was added dropwise with heating to keep the temperature between145° C. and 150° C. Meanwhile, water and ammonia were recovered at theoutlet of the cooler, together with some solvent.

After 128 g of solution was added was added to the decanol suspension,suddenly a sticky mass appeared. Addition of some 80 g of aspartic acidcrystals restored the solid suspension. The mixture was further heatedto remove some residual water and ammonia.

The aspartic acid crystals were removed by vacuum filtration. Afterwashing with ethanol and water 186 g of aspartic acid crystals wereobtained with 94% purity (polarimetric assay).

EXAMPLE 2

In the same experimental device as in Example 1, 415 g of decanol and 77g of aspartic acid crystals were heated to about 150° C. Then a solutionof ammonium aspartate (obtained by bioconversion of ammonium fumarateusing Pseudomonas fluorescens), containing 27% W/W of ammoniumaspartate, was added dropwise to the mixture.

The suspension remained homogeneous untill the end of the reaction.Using the same procedure as above, aspartic acid crystals were obtainedwith 986 purity.

EXAMPLE 3

The same trial as in example 2 was reproduced, except that the ammoniumaspartate solution was concentrated to 56% W/W, prior to the addition tothe decanol aspartic acid mixture.

The suspension remained homogeneous until the end of the reaction. Thereaction was stopped after that 148 g of solution was added. Separationand washing was performed as above.

EXAMPLE 4

A 5 l reaction vessel was equipped with a stirrer, an inlet for thecontinuous feeding of aqueous ammonium aspartate, an inlet for decanoladdition, a rectification column and cooler, and an outlet to removedecanol aspartic acid suspension.

In the 5 l reactor 300 g aspartic acid crystals were dispersed in 3 l ofdry decanol. This mixture was then heated under stirring up to 150° C.When this temperature was reached, continuous addition of concentratedpreheated ammonium aspartate solution (60% W/W) was started. After about200 g of the concentrated solution was added, removal of aspartic acidwas started. This was done discontinuously, every time that anadditional 100 g of ammonium aspartate solution was added. This ammoniumaspartate solution was added at a rate of 4 g/min. The decanol asparticacid suspension was filtered and the decanol returned to the reactionvessel. The thus obtained aspartic acid was further converted intopolysuccinimide in a separate reactor.

EXAMPLE 5

In a 500 ml vessel equipped with stirrer, rectification column andcooler, 50 g of aspartic acid, obtained from example 3, was heated indecanol until a constant temperature of 235° C. was reached.

Water evolution started at 210° C., the colour of the crystals turnedfrom white to pink, then salmon orange with completion of the reaction.

The reaction mixture was then cooled to 60-70° C. and crystals ofpolysuccinimide removed by filtration. About 35 g of solid wasrecovered. Nitrogen content of the solid was 13.7%. In theory nitrogencontent of polysuccinimide=14.2% and of aspartic acid=10.4%.

EXAMPLE 6

Example 4 was repeated with 150 g of aspartic acid instead of 50 g.About 150 g of polysuccinimide was recovered having a nitrogen contentof 14.5%.

What is claimed is:
 1. Process for the preparation of aspartic acid by decomposition of ammonium aspartate, characterised in that a concentrated aqueous solution of ammonium aspartate is constantly introduced into a diluent in which water has a solubility of less than 2% by weight, which diluent is maintained at a temperature at which ammonium aspartate decomposes into aspartic acid and ammonia, and in that the water and the formed ammonia are continuously removed.
 2. Process according to claim 1, characterised in that the aqueous solution has a concentration of at least 25% by weight of ammonium aspartate.
 3. Process according to claim 1 wherein the solubility of water in the used diluent is less than 2% by weight.
 4. Process according to claim 1 wherein the solubility of water in the used diluent is less than 1%.
 5. Process according to claim 1 wherein the diluent is maintained at a temperature above 130° C.
 6. Process according to claim 1 wherein the diluent is maintained at a temperature between 130 and 160° C.
 7. Process according to claim 1 wherein the diluent is maintained at a temperature between 140 and 150° C.
 8. Process according to claim 1 wherein the diluent has a boiling point of at least 165° C.
 9. Process according to claim 1 wherein the diluent has a boiling point of between 190 and 300° C.
 10. Process according to claim 1 wherein the diluent is selected from higher alcohols, higher aliphatic hydrocarbons, higher aromatic hydrocarbons, higher alkyl ethers and high boiling aromatic ethers.
 11. Process according to claim 1 wherein the diluent is selected from decanol, dodecanol, decaline, tetraline, and phenyl linear or branched C₁₀-C₂₀ alkanes.
 12. Process according to claim 1 wherein the rate of introduction of the aqueous ammonium asparate solution and/or the rate of removal of the water, ammonia and aspartic acid is such that the formation of a two-phase system of two liquids is prevented.
 13. Process according to claim 1 wherein the content of aspartic acid crystals in the reaction medium during the continuous process is at least 10% by weight of the amount of diluent used.
 14. Continuous process for preparing polysuccinimide starting from fumaric or maleic acid and ammonia, characterised by the following process steps: reaction of fumaric or maleic acid, or salts thereof, with ammonia to prepare ammonium fumarate or maleate, continuous bioconversion of the ammonium fumarate or ammonium maleate respectively into an aqueous ammonium aspartate solution, using immobilised aspartase or fumarate isomerase/aspartase respectively containing micro-organisms, concentrating the aqueous solution, continuously introducing the concentrated aqueous solution of ammonium aspartate into a diluent in which water has a solubility of less than 2% by weight, which diluent is maintained at a temperature at which ammonium aspartate decomposes into aspartic acid and ammonia, continuously removing water and the formed ammonia and aspartic acid at a rate substantially corresponding to the rate of introducing the ammonium aspartate solution into the diluent, whereas the ammonia is recovered for use in the conversion of fumaric or maleic acid into ammonium fumarate or maleate, thermal polycondensation of the obtained aspartic acid into polysuccinimide.
 15. Process according to claim 14, wherein the solubility of water in the diluent is less than 2% by weight.
 16. Process according to claim 14, wherein the solubility of water in the diluent is less than 1%.
 17. Process according to claim 14, wherein the diluent is maintained at a temperature above 130° C.
 18. Process according to claim 14, wherein the diluent is maintained at a temperature between 130 and 160° C.
 19. Process according to claim 14, wherein the diluent is maintained at a temperature between 140 and 150° C.
 20. Process according to claim 14, wherein the diluent has a boiling point of at least 165° C.
 21. Process according to claim 14, wherein the diluent has a boiling point between 190 and 300° C.
 22. Process according to claim 14, wherein the diluent is selected from the group consisting of higher alcohols, higher aliphatic hydrocarbons, higher aromatic hydrocarbons, higher alkyl ethers and high boiling aromatic ethers.
 23. Process according to claim 14, wherein the diluent is selected from the group consisting of the diluent is selected from decanol, dodecanol, decaline, tetraline, and phenyl linear or branched C₁₀-C₂₀ alkanes.
 24. Process according to claim 14, wherein the rate of introduction of the aqueous ammonium aspartate solution and/or the rate of removal of the water, ammonia and aspartic acid is such that the formation of a two-phase system of two liquids is prevented.
 25. Process according to claim 14, wherein the content of aspartic acid crystals in the reaction medium during the continuous process is at least 10% by weight of the amount of diluent used. 